It seams that webmin users groups do not work as expected.
I'm trying to set user group with couple of available modules and some of them with custom access control options. For example: enable: postfix server module and disable all options in it except "Can manage mail queue?".
But when I add a user to the group the options set in the modules are not actually set i.e. anything is shown when in the group is set to show only couple of them.
A workaround that I found is to go to every user and just save the user without changing anything.
The issue exist also when you try to do the same trough "Cluster Webmin Servers" from the Manage server.